Re: Interesting C++ way to allocate user data (placement new)

[Matthias Kluwe <mkluwe@...>]

Hi!

2012/5/16 Pablo Pissanetzky <pablo@trickplay.com>:
> Just wanted to share this idea:
>
> void * operator new( size_t size , lua_State * L )
> {
>   return lua_newuserdata( L , size );
> }
>
> Then:
>
> Foo * foo = new ( L ) Foo( constructor arguments );

"Placement new" works without overloading operator new. It's a matter
of taste if you wish to save a few keystrokes here.

Doing this depends on the returned memory being properly aligned to
hold an object of type Foo. Does anyone have a hint why this can be
assumed?

In the past I've fallen back on

Foo **f = (Foo**) lua_newuserdata( L, sizeof( Foo* ) );
*f = new Foo();

(Of course, this assumes the returned memory can hold a pointer...)

> And in __gc you have to:
>
> ( static_cast< Foo * >( lua_touserdata( L , 1 ) ) )->~Foo();
>
>
> It looks elegant but also dangerous ( in a good way, I think :)
>
> Pablo
>
> P.S. You also have to have a delete operator to deal with exceptions.

You mean, you have to deal with ~Foo() throwing exceptions? That's not
the case, hopefully (see
http://www.parashift.com/c++-faq-lite/exceptions.html#faq-17.9).

Regards,
Matthias